The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Lithium-Ion Batteries have become the battery of choice for electric vehicles and high performance hybrid electric vehicles. However, despite the many advantages provided by such batteries, various cost, safety, and performance issues remain. For example, as most lithium ion batteries use graphite in the anode as intercalation host, several problems arise: The low storage voltage of Li/Li+ for graphite tends to result in lithium plating on the graphite surface, especially during poorly managed charging cycles, leading to cell shorting and in some cases fire. Further, the low storage voltage in graphite also causes electrolyte decomposition and the formation of a passivating solid electrolyte interphase on the graphite surface, which consumes significant quantities of lithium and as such substantially reduces energy density. Moreover, the slow Li3+ ion transfer in the interface between the solid electrolyte interphase and the graphite often further exacerbates the risk of lithium plating and fires. Finally, and in addition to graphite being an excellent fuel for battery fires, graphite will not adhere directly to metal foil current collectors and must therefore be bound to the surface of the collectors with polymers. The thusly formed polymer matrices have several intrinsic limitations, including resistivity, relatively poor current distribution, and distortion/cracking under cycling.
Inexpensive, abundant, noncombustible, and non-toxic titanium dioxide (TiO2) has been considered by many as an attractive candidate as an intercalation host for Li ions in lithium ion batteries. Advances in the understanding of the mechanism by which Li+ intercalates and/or attaches to the surface of TiO2 nanoparticles have been made in a number of applications outside of batteries. For example, dye sensitized solar cells with TiO2 nanostructures have been reported and it has been reported that nanostructures of TiO2 can be optimized for Li+ intercalation by intercalation within the crystalline structure and/or surface attachment (see e.g., Hairima Y, et al. Improvement of photovoltages in organic dye-sensitized solar cells by Li intercalation in particulate TiO2 electrodes. Appl. Phys. Lett. 2007; 90:103517-103519).
TiO2 can advantageously be sintered directly to the surface of a range of metal foils without a polymer binder, and such coatings tend to have an improved resistance to abrasion, cracking, and separation. Alternatively, EU 1244168 describes methods for attaching films of mesoporous TiO2 and other metal oxides to the surface of metal foils by means of a polymer network that passes through the mesoporous structures rather than adhering to their surface (and thus inhibiting conductivity). As a result, high throughput “roll to roll” coating of mesoporous TiO2 is now possible and relatively inexpensive. Unfortunately for all mesoporous forms of TiO2 (anatase, rutile, and more recently TiO2(B)), the usefulness of TiO2 coated foils is rather limited due to the insulating properties of TiO2's crystalline structure, allowing only for very modest power densities, typically not exceeding a few mW/cm2. While such power densities are often adequate for dye-sensitized solar cells applications, they are not sufficient to warrant interest in high power and/or high current density applications.
Magneli phase sub-oxides of titanium (MPST) are similar to TiO2, but are electrically conductive, chemically inert, cannot burn, and will not participate in thermal runaway reactions. MPSTs are of the formula TinO(2n-1) where n is between 3 and 10. Unfortunately, Ti3O5 has been found to be unstable in oxidizing conditions. Consequently, Magneli phases of titanium oxide that are commercially useful, are the phases where n lies in the range of 4 to 10, inclusive. It is generally believed that MPSTs derive their properties from a structure in which layers of two dimensional chains of octahedral TiO2 are separated by layers in which oxygen atoms are missing. These oxygen deficient layers are known as Magneli shear planes (MSP). The combination of electrical conductivity and similarity to TiO2, have made MPSTs attractive for high performance electrodes in a number of advanced energy devices, including ozone generation, electrochemical reduction/oxidation reactions, air electrodes for metal air batteries, and highly stable catalyst supports for PEM fuel cells. The stability and performance advantages of monolithic MPSTs are well documented in these applications.
The most conducting MPST is Ti3O5, which has an MSP, at every 3rd layer. The most researched MPST is Ti4O7 which is stable in oxidizing conditions has an MSP, at every 4th layer and is approximately 2.7 times more conductive than graphite. The last useful and least electrically conductive phase is Ti10O19 which has a MSP every 10th layer and an electrical conductivity approaching that of TiO2. Importantly, the surface structure of MPSTs are predominantly that of blocks octahedral TiO2, separated by the edges of the MSPs. Consequently, Ti3O5 has the lowest ratio of octahedral TiO2 to MSP and Ti10O19 has the highest.
Notably, one important limitation of MPSTs is that, at the nanoscale, the surface layers are susceptible to oxidation and will revert back to the electrically insulating TiO2. Although research into this phenomenon is incomplete, instability appears to be greatest in Ti3O5 and Ti4O7, and least in Ti10O19. At the micron scale, this limitation is insignificant and monolithic MPSTs are attractive and proven in many applications. However, from a practical perspective, conventional MPSTs are unstable as nanostructures and as nanoscale surface features (such as the mesoporous structures desired for use as high surface area electrodes and catalyst supports). Research into conventional MPSTs as an alternative to graphite in lithium ion batteries has so far been limited to the poorly conductive MPSTs Ti9O17 and Ti10O19, coated with a protective layer of carbon to protect the MPST from oxidation. The results of such attempts were predictably mixed, reflecting the compromises inherent in the material.
In another approach (J. Electrochem. Soc. 2002, Volume 149, Issue 8, A1092-A1099), a fully oxidized Magneli Phase material was created that included an element of dissimilar size to Ti to thereby distort the crystal lattice into a structure in which crystallographic shear planes were present. While niobium and tantalum were shown to increase oxidation resistance, their expense and scarcity presents a significant limitation to this approach. Likewise, other suitable dissimilar elements for such use were high cost rare earth metals.
Thus, even though numerous materials are known to produce electrode materials with desirable properties, all or almost all of them suffer from one or more disadvantages. Therefore, there is still a need for improved systems, compositions, and methods to produce a highly stable and corrosion resistant electrode material.